The Third Generation Partnership Project (3GPP) has begun on work on the development and design of the next generation mobile communications system (a.k.a., the 5G mobile communication system or the Next Generation (NG) system or simply “5G” or “NG” for short). 3GPP is currently in the process of specifying the 5G radio interface called New Radio (NR) or “G-UTRA” as well as a Next Generation Packet Core Network (NG-CN or NGC).
A user equipment (UE) (i.e., a device capable of wireless communication with a radio access network node, such as, for example, a smartphone, a laptop, a tablet, a smart sensor, an appliance, etc.) needs to register with a network in order to receive services which require registration. A registration procedure is utilized in the following situations: (1) when the UE initially registers to a 5G system; (2) when the UE initiates a registration procedure due to mobility, e.g. when the UE changes to a new Tracking Area (TA) in idle mode; (3) when the UE performs a periodic update due to expiration of a predefined time period of inactivity, etc.
As part of the 5G work in 3GPP, a network slice has been described as a logical network that provides specific network capabilities and network characteristics, and a network slice instance as a set of network function instances and the required resources (e.g. compute, storage and networking resources) which form a deployed network slice. Further details on network slicing are described in 3GPP TS 23.501, v1.0.0 (2017-06) “3rd Generation Partnership Project; Technical Specification Group Services and System Aspects; System Architecture for the 5G System; Stage 2” (Release 15) (“3GPP TS 23.501”), and specifically clause 5.15 Network Slicing.
By way of background:                Network slices may differ for supported features and network functions optimisations. The operator may deploy multiple Network Slice instances delivering exactly the same features but for different groups of UEs, e.g. as they deliver a different committed service and/or because they may be dedicated to a customer.        A single UE can simultaneously be served by one or more Network Slice instances via a 5G-AN. A single UE may be served by at most 8 Network Slices at a time. The AMF instance serving the UE logically belongs to each of the Network Slice instances serving the UE, i.e. this AMF instance is common to the Network Slice instances serving a UE.        (3GPP TS 23.501, clause 5.15.1).        
A S-NSSAI (Single Network Slice Selection Assistance information) identifies a Network Slice, and a NSSAI is a list of S-NSSAIs. Further, as provided in 3GPP TS 23.501, clause 5.15.2.1:                The (R)AN may use Requested NSSAI in access stratum signalling to handle the UE Control Plane connection before the 5GC informs the (R)AN of the Allowed NSSAI. The Requested NSSAI is not used by the RAN for routing when the UE provides also a Temporary User ID.        . . .        When a UE is successfully registered, the CN informs the (R)AN by providing the whole Allowed NSSAI for the Control Plane aspects.        
Each S-NSSAI may have a standardized value or a public land mobile network (PLMN) specific value. Where the S-NSSAI has a PLMN-specific value, the PLMN-specific value is associated to a PLMN ID of the PLMN that assigns it. (This construct may be similar to the existing parameter UE Usage Type from 3GPP Rel-13 and the 4G LTE access.)
Regarding the storage of NSSAI (collection of S-NSSAIs) in the UE, clause 5.15.4 of TS 23.501 gives the following:                A UE can be configured by the [home PLMN] HPLMN with a Configured NSSAI per PLMN. A Configured NSSAI can be PLMN-specific and the HPLMN indicates to what PLMN(s) each Configured NSSAI applies, including whether the Configured NSSAI applies to all PLMNs, i. e. the Configured NSSAI conveys the same information regardless of the PLMN the UE is accessing (e.g. this could be possible for NSSAIs containing only standardized S-NSSAIs). When providing a Requested NSSAI to the network upon registration, the UE in a given PLMN shall only use S-NSSAIs belonging to the Configured NSSAI, if any, of that PLMN. Upon successful completion of a UE's Registration procedure, the UE may obtain from the AMF an Allowed NSSAI for this PLMN, which may include one or more S-NSSAIs.        . . .        For each PLMN, the UE shall store the Configured NSSAI and, if any, the Allowed NSSAI. When the UE receives an Allowed NSSAI for a PLMN, it shall store it and override any previously stored Allowed NSSAI for this PLMN.        
Particularly for roaming the TS 23.501 clause 5.15.6 stipulates that:                For roaming scenarios, the Network Slice specific network functions in VPLMN and HPLMN are selected based on the S-NSSAI provided by the UE during PDU connection establishment as following.        If a standardized S-NSSAI is used, then selections of slice specific NF instances are done by each PLMN based on the provided S-NSSAI;        Otherwise, the VPLMN maps the S-NSSAI of HPLMN to a S-NSSAI of VPLMN based on roaming agreement (including mapping to a default S-NSSAI of VPLMN). The selection of slice specific NF instance in VPLMN are done based on the S-NSSAI of VPLMN, and the selection of any slice specific NF instance in HPLMN are based on the S-NSSAI of HPLMN.        